Experimental Infection of Mink with SARS-COV-2 Omicron Variant and Subsequent Clinical Disease

We report an experimental infection of American mink with SARS-CoV-2 Omicron variant and show that mink remain positive for viral RNA for days, experience clinical signs and histopathologic changes, and transmit the virus to uninfected recipients. Preparedness is crucial to avoid spread among mink and spillover to human populations.

S ARS-CoV-2 has been detected in farmed and feral American mink (Neovison vison) in multiple countries, and extensive environmental contamination and human-to-mink and mink-to-human transmission has been documented (1)(2)(3)(4)(5). These factors have led to strict measures in mink farms and mink-farming countries to prevent the spread of the disease. In late 2021, a new SARS-CoV-2 variant (Omicron), characterized by possibly milder symptoms and more efficient human-tohuman transmission, was detected, but its infectivity and spread in American mink is unknown (6,7).
We tested the response of American mink to the Omicron variant by infecting 3 male mink intranasally with 4 × 10 5 plaque-forming units of the virus (Appendix, https://wwwnc.cdc.gov/EID/ article/28/6/22-0328-App1.pdf). We conducted follow-up on infected mink for 7 days and performed histopathologic evaluation of upper and lower respiratory tracts on the last day of follow-up. We sampled saliva daily.
All experimentally infected mink showed mild to moderate signs of illness, including lethargy, anorexia, diarrhea, nasal and lacrimal discharge, and sneezing. Consistent with earlier experiments with other variants (8; D. Adney et al., unpub. data, https:// www.biorxiv.org/content/10.1101/2022.01.20.47716 4v1), saliva samples tested PCR-positive 1 day postinfection (dpi) and remained that way throughout follow-up (Table; Appendix). Infectious virus was cultured 1-3 dpi. Even though some of the clinical signs could be caused by other factors, such as stress from the change of environment, their consistency with signs seen in studies of other variants, combined with PCR results, demonstrate that the Omicron variant also causes clinical disease in mink.
To study whether mink can transmit the virus, we placed 2 uninfected indirect contact mink in separate cages 10-20 cm from the cages of the infected mink and followed their progress for 10 days. Similar signs to the experimentally infected mink developed in both initially uninfected mink, and they were consistently PCR-positive from day 3 onward (Table), indicating mink-to-mink transmission. Infectious virus was detected in cell culture even before it was detected by PCR. Even though no evidence of mink-to-human transmission of the Omicron variant exists, it seems possible on the basis of our results and the information from studies of other variants.
Gross findings in the nasal cavity and lungs were subtle in both experimentally infected and recipient mink and consisted of hyperemia of respiratory mucosa with small amounts of viscous exudate and noncollapsed, dark-red, and wet pulmonary lobes. All mink showed histopathologic changes in the upper and lower respiratory tracts ( Figure). We observed multifocal degeneration and loss of respiratory epithelium with variable mucosal and submucosal neutrophilic infiltration in the nose. The lumen contained sloughed epithelial cells, mucinous material, and degenerated neutrophils ( Figure, panels A, C). Viral nucleoprotein was widely distributed beyond intact cells, within sloughed cells, and in mucosal respiratory epithelium (Figure,

Table.
Results of PCR and cell culture testing for SARS-CoV-2 Omicron variant in saliva samples from 3 experimentally infected mink and 2 uninfected recipient mink* Mink ID 1 dpi 2 dpi 3 dpi 4 dpi 5 dpi 6 dpi 7 dpi 8 dpi 9 dpi 10 dpi intra-alveolar macrophages, few syncytial cells, bronchi and bronchiolar epithelial cell hyperplasia, vasculitis, and perivasculitis. We could not detect viral antigen in this mink. Strikingly, all evaluated mink lacked viral antigen in the epithelium of bronchi and bronchioles. The Omicron variant is different from other variants in its more efficient spread, primarily attributable to immune escape and likely milder symptoms in humans (6,7). These factors make preventing virus introduction into mink farms through asymptomatic humans more difficult, creating a more substantial risk for the formation of virus reservoirs among farmed or feral mink. This study shows that mink can be infected by Omicron and, crucially, efficiently transmit the virus to other mink. Despite the reports of lower virulence of Omicron, mink experience clinical disease and nasal and pulmonary microscopic lesions that closely resemble infection with previously reported variants in mink and humans (8). Clarifying the clinical signs will help detect the virus among mink earlier. Questions remain about the risks that the spread of this easily transmitted variant among mink would create for public health, including transmission to humans and emergence of mink-specific mutations, followed by their spillover to human population. This article was preprinted at https://www.biorxiv.org/ content/10.1101/2022.02.16.480524v2.